Control system for polymerization reactors

ABSTRACT

The flow of catalyst to a polymerization reactor is controlled in response to a computation of the polymer production rate. A signal representative of the computed production rate is passed through a gated derivative controller and divided by a signal representative of the catalyst productivity. The quotient is employed to modify the computer production rate control signal.

United States Patent [151 3,636,326

Smith et al. 1 51 Jan. 18, 1972 1541 CONTROL SYSTEM FOR [56] 1161mmc1160 POLYMERIZATION REACTORS UNITED STATES PATENTS [721 lnvenm's P'wmim bmh 3,078,265 2/1963 Berger et a1 ..235/151.12 x 3,533,236 10/1970Cottington ..3l8/609 x [73] Assignee: Phillips Petroleum Company [22]Filed: July 24, 1970 [21] Appl. No.: 58,041

Primary Examiner-"Eugene G. Botz Attorney-Young & Quigg '11 ABSTRACT 52us. (:1 ..235/151.12, 260/949 P, 260/95, The catalyst to Polymrizationreactor is l ed 318/610 in response to a computation of the polymerproduction rate. 511 1111.01 ..G06g 7/53, (10513 11/42, BOlj 9/00 Asignal representative of the computed production te is 581 Field 61Search .235/151, 12; 318/609, 610; P through a sated derivativecontroller and ded by a 260,949 P signal representative of the catalystproductivity. The quotient is employed to modify the computer productionrate control signal.

6 Claims, 2 Drawing Figures a7 1 as H 93 PR DUCTION RATE SE POINTINTEGRATOR PRODUCTION RATE COMPUTER POLYMER WITHDRAWAL DENSITY OFWITHDRAWN POLYMER CATALYST FLOW RATE PATENTED JAN] 8 m2 sum 1 or 2INVENTORS D. E SMITH W. S. STEWART N mzuiIE .r m on J 9 mm mm Mmufiozozou mm 9 kzunia mm mm A T TORNE VS PATENTED JAM a 972 SHEET 2 [1F2 INVENTORS D.E. SMITH W.S STEWART mm mm A T TOR/V5 Y5 CONTROL SYSTEMFOR POLYMERIZATION REACTORS g In many chemical operations it isdesirable to carry out reactions at'substantially uniform rates in orderto produce specification products at minimum operating costs. One suchexample occurs in the production of olefin polymers in continuousreactors with fresh catalyst being added as may be required. In such asystem it is often difficult to control the rate of catalystintroduction so that polymerization takes place at a uniform rate. Thisis duein part to the fact that catalyst poisons may be present invarying amounts in one or more of the feed streams supplied to-thereactor. In addition,. some types of catalysts do not immediatelyproduce polymer when introduced into a reactor. A certain inductionperiod must be overcome before the full production rate is established.While various types of automatic control systems have been devised foruse with reactors of this type, it is still difiicult to maintainpolymer production at uniform rates. A v

In accordance with one aspect of this invention, an improved controlsystem is provided which can utilize a gated derivative mode-of control.As applied to the control of a polymerization reaction, for example, thebasic control is based on a computation of the production rate withinthe reactor. The output signal from the production rate computer isutilized to regulate the introduction of fresh catalyst into thereactor. The gated derivative control mode of this invention is employedto compensate for a rate of change of reaction being above a criticalvalue or'abovea desired percentage of the production rate set pointvalue. This gated derivative mode of control serves to adjust thecatalyst introduction rate so as to maintain a uniform production rateof polymer. In another aspect of this invention, an improved controlsystem fora polymerization process is provided.

In the accompanying drawing:

FIG. 1 is a schematic representation of a polymerization reaction systemhaving the control system of this invention incorporated therein. l v

FIG. 2 is a schematic drawing of the computer employed in the controlsystem of FIG. 1.

Referring now to the drawing indetail and to FIG. 1 in particular, thereis shown a conventional reactor .10 which is employed to produce olefinpolymer. In the specific embodiment to be described, a copolymer ofethylene with another olefin, such as hexene-l, is produced by a processof the type described in US. Pat. No. 2,825,721. Ethylene is introducedinto reactor 'through a conduit 11 which has a flow controller 12associated therewith to adjust a valve 13. The ethylene is thusintroduced at a predetermined rate in accordance with a set pointvalueapplied to controller 12. A comonomer, such ashexene-l,isintroduced through aconduit 14 which has a flow controller 15associated-therewithto adjust a valve 16. Conduits 11 and 14 join acommon conduit 17 which communicates with reactor 10. A diluent, such asisobutane or normal butane, is introduced through a conduit 18 which hasa flow controller 19 associated therewith to regulate a valve 20.Catalyst is introduced through a conduit 21 which has a control element22 therein. Control element 22 can be a rotatable valve which introducesa catalyst slurry into the system in increments, the rate of catalystintroduction being controlled by the speed of rotation of a motor 23which actuates valve 22. This controls the dump frequency of the valve.a a

Reactor 10 is provided with animpeller 24 which is rotated by a motor2510 circulate the contents of the reactor around aclosed loop. Theproduced polymer settles into a plurality of takeofi' legs 26, 27 and 28which have respective control valves 26a, 27a and 28d therein. Conduits26, 27 and 28 communicate at their downstream ends with a productremoval conduit 30. Reactor 10 is provided with jackets 31 through.which a heat-exchange fluid is circulated. The-polymerization is anexothermic reaction so that it is usually necessary to pass a coolantthrough the jacket to remove heat. This coolant flows in a closed pathwhichincludes a withdrawal conduit 32 having a pump, not shown, therein.Conduit 32 communicates with parallelconduits 33 and 34 which have acooler 35 and a heater 36, respectively, therein. The twoparallelconduits join at a conduit 37 which introduces the fluid into jacket 31.Control valves 38, 39 and 40 are positioned in respective conduits33,34- and 37. Valve 40 is controlled by a flow controller 41 whichmaintains a constant flow of coolant through the system. Valves 38 and39 are regulated by a-temperature controller 42 which responds to atemperature transducer 43that measures temperature within the reactor.Valves 38 and 39 operate in opposition to control the relativeflowsthrough cooler 35 and heater 36. The temperature of theheatexchange fluid is thus regulated to tend to maintain a constanttemperature within the reactor, as sensed by transducer 43.

In accordance with one embodiment of this invention, the rate ofintroduction of catalyst into the reactor is adjusted by a motor speedcontroller 45 which receives'a signal from a computer 46. The outputsignal from computer 46 serves to adjust thespeed of motor 23, and thusthe rate of catalyst addition. As described in greater detailhereinafter, computer 46 receives a plurality of input signals'whichrepresent variables of the reaction system of FIG. '1. Computer 46includes a production rate computer which computes the rate of polymerproduction in response -to a measurement of the heat of polymerization.To this end, computer 46 receives signals which represent variables ofthe'system. A pressure transducer 50 and a temperature transducer 51measure the pressure and temperature of the ethylene in conduit 11 andestablish signals at respective terminals 52 and 53 representative ofthese conditions. These signals are applied to computer 46. Similarly, asignal is established at a terminal 54 which represents the volumetricrate of flow of ethylene through conduit 11. A temperature transducer 55establishes a signal at a terminal 56 which is representative of thetemperature of the liquid comonomer. A signal is established at terminal57 which represents the volumetric rate of flow of the comonomer. Ifthis comonomer is present as a gas, a pressure transducer is alsoemployed. In similar fashion, a temperature transducer 58 establishes asignal at a terminal 59 which represents the temperatureof the diluent.A signal is established at a terminal 60 which represents the volumetricflow of diluent. A signal representative of the output of controller 45is established at a terminal 61. This signal represents the'speed ofrotation of motor 23 and thus the actualrate of catalyst addition.

Signals representative of the temperatures of the heat exchangefluidenteringand leaving jacket 31 are established at terminals 63 and 64 byrespective temperature transducers 65 and 66. A signal representative ofthe rate of flow of coolant is established at a terminal 67. The outputsignal of temperature transducer 43 is applied to a terminal 68. Adensitymeasuring'device 69 is associated with reactor 10 to establish anoutput signal at a terminal 70 which is representative of the density ofthe material within the reactor. This density measuring device can be agamma ray detector, for example. The

attenuation of a stream-of'gamma rays passed through the reactor froma'source, not shown, is thus a=function of the density of the materialwithin the reactor.- Similarly, a densitymeasuring device 71 establishesa signal at a terminal 72 which is representative of the density of thepolymer slurry collected'in settling leg 26, from which the polymerconcentration is readily calculated by suitable calibration of themeter. The rate of withdrawal of polymer from the reactor is regulatedby a controller 73 which operates valves 26a, 27a and 28a in response toa measurement of the pressure of the monomer near the introduction pointinto the reactor. Controller 73 can operate the dump valves in sequence.A pressure transducer 74 establishes a signal at terminal 75 which isrepresentative of this measured pressure. Controller 73 establishes asignal at a terminal 76 which is representative of the actual rate ofpolymer slurry withdrawal. A signal is established at a terminal 77which is representative of the power input of motor 25'and thus of theamount of energy imparted to the moving stream within reactor 10. Fromthese input signals an overall heat balance can be made and the polymerformation rate can be calculated.

Many of the output signals from the system of FIG. 1 are applied to aproduction rate computer 80 which is shown in FlG. 2. This computerestablishes an output signal which is representative of the rate ofproduction of polymer within reactor 10. Such a computer can conduct aheat balance computation and can be of the type described in U.S. Pat.Nos. 2,974,017 and 3,078,265, for example. The output signal fromcomputer 80 is applied to a summing device 81 and to the input of adifferentiating device 82. The second input signal to summing device 81is obtained from a summing device 83, as described hereinafter. Theoutput signal from device 81 is applied to the first input of a summingdevice 84. A reference signal representative of the desired productionrate of polymer within reactor is applied from an input terminal 85 tothe second input of device 84. The output of summing device 84 isapplied to the input of an integrator 86 and to the first input of amultiplier 87. A constant signal 88 constitutes the second input tomultiplier 87. The output of integrator 86 is applied to the first inputof a multiplier 89. A constant signal 90 constitutes the second input tomultiplier 89. The output signals from multipliers 87 and 89 and setpoint signal 85 are applied to the three inputs of a summing device 91.The output signal from device 91 is applied as the numerator to a signaldivider 92. The output signal from divider 92 is applied to the firstinput of a summing device 93. The output of device 93 is applied to thefirst input of a summing device 94. The output of device 94 is appliedto an output terminal 95. The signal of terminal 95 is applied as theinput to controller 45 of FIG. 1.

A signal representative of the polymer withdrawal rate from the reactoris applied from terminal 76 to the first input of a signal multiplier98. As previously mentioned, the signal applied to terminal 76represents the rate of withdrawal of polymer slurry through the settlinglegs. Terminal 72 is connected to the second input of multiplier 98. Thesignal applied to terminal 72 is representative of the density of thepolymer slurry withdrawn through the settling legs, thereby its polymerconcentration. The output signal from multiplier 98 is thusrepresentative of the mass rate of polymer withdrawal from the reactor.This signal is applied as the numerator to a signal divider 99. Thesignal at terminal 61, which is representative of the catalyst flow rateinto reactor 10, is applied to the first input of a summing device 100.The output of divider 99, which is representative of catalyst effluentrate, is connected to the second input of device 100. The output signalfrom device 100 is applied to the input of an integrator 101. The outputof the integrator is applied as the denominator to a signal divider 102.The signal representative of the polymer mass within reactor 10 isapplied from terminal 70 as the numerator to divider 102. This signal isobtained from the density-measuring device 69, the output of which iscalibrated to represent the mass of polymer within the reactor or as abasis for calculating the mass of polymer within the reactor. The outputsignal from divider 102 is applied as the denominator to signal divider92. This same signal, which represents the catalyst produ lb polymerfive , is also applied as the denominator to divider 99. The outputsignal from summing device 100 is also applied to the first input of amultiplier 104. A constant signal 105 constitutes the second input tomultiplier 104. The output signal from multiplier 104, which representsthe rate of catalyst accumulation within the reactor, is applied to thesecond input of summing device 93. I

The output signal from divider 102 is applied to the first input of asumming device 106. A constant signal 107 constitutes the second inputto device 106. The output of summing device 106 is applied to the firstinput of a multiplier 108. Terminal 61 is connected to the second inputof multiplier 108. The output signal from multiplier 108 is connecteddirectly to the firstjnput of a summing device 83 and through a lagmeans 109 to the second input of summing device 83. As previouslymentioned, the output of device 83 is connected to summing device 81.

The output signal from production rate computer is applied to the inputof signal comparing means 112. A constant signal 113 is connected as thesecond input to comparing means 112. The first output of comparing means112 is removed from the circuit, while the second output is connected tothe input of differentiating device 82.

The output signal from derivative device 82 is applied to the input of asignal-comparing means 114. A constant signal 115 is also connected tocomparing means 1 14. The first output of comparing means 114 is removedfrom the circuit, while the second output is connected as the numeratorto a signal dividing means 118. The output signal from divider 102 isapplied as the denominator to a divider 1 18. The output of divider 118is connected to the first input of a signal multiplier 116. A constantsignal 117 constitutes the second input of multiplier 116. The output ofmultiplier 116 is connected to summing device 94.

As previously mentioned, the control system is utilized to maintain theproduction rate as nearly uniform as possible. The signal fromproduction rate computer 80 is the basic control signal which regulatesthe rate of catalyst addition. The control elements including multiplier87, summing device 84, integrator 86 and summing device 91 provide bothproportional and integral modes of control. Device 84 compares thecomputed production rate with the desired production rate for a givensystem. Any difference between the two rates is applied throughintegrator 86 to device 91. Multipliers 87 and 89 are employed toincorporate suitable scale factors as may be required to maintain propercatalyst flow rates in a given system. The values of the variousconstant signals can be obtained by routine trial and error proceduresto provide smooth and relatively accurate control in any given system.

Another factor involved in the control of catalyst introduction is thecatalyst accumulation within the reactor. To this end, the output signalfrom multiplier 98 is representative of the mass rate of withdrawnpolymer. As will become apparent, the output signal from divider 99 isrepresentative of the flow of catalyst out of the reactor along with thepolymer product. Catalyst is removed at a certain flow rate in thismanner. This signal is subtracted from the flow of catalyst into thereactor by device 100 to obtain a signal representative of the rate ofcatalyst accumulation in the reactor. This signal is integrated byintegrator 101. In divider 102, the mass of polymer within the reactoris divided by the accumulated catalyst to provide a signalrepresentative of catalyst productivity. The quotient is the denominatorin divider 99, thereby providing the previously mentioned output signal.The catalyst flow input signal from terminal 61 is multiplied by theoutput signal from divider 102, after having a constant inserted bydevice 106, and the product is transmitted through lag means 109. Lagmeans 109 and device 83 provide dead time compensation. As is wellknown, a change in catalyst flow does not immediately change theproduction rate. This compensation is inserted at device 81. Multiplier108 and lag means 109 operate in the same general manner as the devicedescribed in U.S. Pat. No. 3,175,764. The output signal from summingdevice 91 is divided by the output signal from divider 102 to provide anadaptive feature of variable gain in the production rate control system.

The computed production rate signal from computer 80 is passed tocomparing means 112. In some applications it is desirable to employ anoise filter between computer 80 and comparing means 1 12. Theproduction rate signal is compared in means 112 with a lower limitsignal 113. If the production rate signal is less than the lower limit,the resulting output is removed from the circuit and no further actionis taken. If the computed production rate signal is greater than lowerlimit 113, it is passed through differentiating device 82 so that asignal representative of the derivative of the production rate withrespect to time is passed to comparing means 114. This signal is thereincompared with a lower limit signal 115. If the derivative signal is lessthan this lower limit 115, the resulting output is removed from thecircuit and no further control action is taken. If the derivative signalis greater than the lower limit 115, the derivative signal is passed todivider 118 wherein it is divided by the output signal from divider 102.The quotient is passed through a scaling multiplier 116 to summingdevice 94. Thus, a derivative mode of control is added when both theproduction rate and its derivative are above respective limits. Theforegoing example illustrates one of the several combinations ofproduction rate limits and limits on magnitude and direction of changewherein a control action is taken. The computer of FIG. 2 can beconstructed of conventional analog elements, either electrical orpneumatic, for example. As an alternative, the input signals can beconverted to digital form and applied to a conventional digital computerwhich is programmed to perform the calculations illustrated in thedrawing.

While this invention has been described in conjunction with presentlypreferred embodiments, it obviously is not limited thereto. What isclaimed is: I 1. In a polymerization system which includes a reactor,means to introduce monomer into the reactor, means to introduce catalystinto the reactor, means to cool the reactor, and means to withdrawpolymer from the reactor; a control system comprising:

means to compute the polymer production rate within the reactor andestablish a first signal representative thereof;

means to compare said first signal with a constant signal representativeof a predetermined production rate and establish a control signalrepresentative of any difference therebetween;

first signal comparing means to compare said first signal with apredetermined first lower limit constant signal and to pass said firstsignal when it is greater than said lower limit constant signal;

derivative means responsive to the first signal passed by said firstsignal comparing means to establish a second signal representative ofthe derivative of said first signal;

second signal comparing means to compare a second signal passed by saidfirst comparing means and derivative means with a predetermined secondlower limit constant signal and to pass said second signal when it isgreater than said second lower limit constant signal;

means to compute catalyst productivity within the reactor and establisha third signal representative thereof;

means to divide a second signal passed by said second comparing means bysaid third signal to establish a fourth signal representative of thequotient;

means to modify said control signal by said fourth signal to establish afifth signal; and

means responsive to said fifth signal to control the rate of catalystintroduction into said reactor.

2. The control system of claim 1 wherein said means to compute catalystproductivity comprises:

means to measure the rate at which polymer is withdrawn from the reactorand establish a sixth signal representative thereof;

means to establish a seventh signal representative of the density of thepolymer withdrawn from the reactor;

means to multiply said sixth signal by said seventh signal and establishan eighth signal representative of the product;

means to measure the rate of catalyst flow into the reactor andestablish a ninth signal representative thereof;

means to divide said eighth signal by said third signal and establish atenth signal representative of the quotient;

means to subtract said tenth signal from said ninth signal and establishan eleventh signal representative of the difference;

means to integrate said eleventh signal to establish a twelfth signalrepresentative of the catalyst within the reactor;

means to measure the amount of polymer within the reactor and establisha thirteenth signal representative thereof; and

means to divide said thirteenth signal by said twelfth signal toestablish said third si al. 3. The control system 0 claim 2 wherein saidmeans to compare said fifth signal with a constant signal comprisesmeans to multiply said third signal by said ninth signal and establish afourteenth signal representative of the product;

means responsive to said fourteenth signal to establish a fifteenthsignal representative of said fourteenth signal delayed by apredetermined amount; means to sum said fourteenth and fifteenth signalsand establish a sixteenth signal of the difference therebetween;difference therebetween;

means to sum said sixteenth signal and said first sigial to establish aseventeenth signal; and

means to compare said seventeenth signal with said constant sigial toestablish said control signal.

4. The control system of claim 1 wherein said means to modify comprisesmeans to divide said control sigral by said third signal and establish asixth representative of the quotient; and

means to combine said sixth signal with said fourth signal and establishsaid fifth signal representative of the resulting difference.

5. The control system of claim 4, further comprising means to establisha seventh signal representative of the rate of change of catalyst withinsaid reactor; and

means to compare said sixth signal with said seventh signal andestablish a signal representative of the difference therebetween, saidlast-mentioned signal being compared with said fourth signal toestablish said fifth signal.

6. The control system of claim 4 wherein said means to establish saidcontrol signal includes means to integrate said control signal, andmeans to sum said constant signal with the resulting integrated signalto establish said control sigial.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION atent No,3,636,326 Dexter E. Smith et al a January 18, 1972 It is certified thaterror appears in the above-identified patent and that said etters Patentare hereby corrected as shown below: 7

Column 6, line 30, representative" has been omitted following "signal";line 31, "difference therebetweem" has been added. following"therebetweeng" I line 38, "signal" has been omitted following "sixth".

Signed and sealed this 18th day of Jul 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents UNITED STATES PATHIT OFFICE CERTIFICATE OF CORRECTION PatentN00 3,636,326 Dexter E. Smith at al Dated= January 18, 197

It is certified that error appears in the above-identified patent andthat s: Letters Patent are hereby corrected as shown below: i

.Celumn 6, line 30, "representative" has been omitted following"signal";

line 31, "difference therebetweem" has been added followingtherebetweeng line 38, "signal" has been emitted following "sixth".

Signed and sealed this 18th da of July 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GQTTSCHALK Commissioner of PatentsAttesting Officer

2. The control system of claim 1 wherein said means to compute catalystproductivity comprises: means to measure the rate at which polymer iswithdrawn from the reactor and establish a sixth signal representativethereof; means to establish a seventh signal representative of thedensity of the polymer withdrawn from the reactor; means to multiplysaid sixth signal by said seventh signal and establish an eighth signalrepresentative of the product; means to measure the rate of catalystflow into the reactor and establish a ninth signal representativethereof; means to divide said eighth signal by said third signal andestablish a tenth signal representative of the quotient; means tosubtract said tenth signal from said ninth signal and establish aneleventh signal representative of the difference; means to integratesaid eleventh signal to establish a twelfth signal representative of thecatalyst within the reactor; means to measure the amount of polymerwithin the reactor and establish a thirteenth signal representativethereof; and means to divide said thirteenth signal by said twelfthsignal to establish said third signal.
 3. The control system of claim 2wherein said means to compare said fifth signal with a constant signalcomprises means to multiply said third signal by said ninth signal andestablish a fourteenth signal representative of the product; meansresponsive to said fourteenth signal to establish a fifteenth signalrepresentative of said fourteenth signal delayed by a predeterminedamount; means to sum said fourteenth and fifteenth signals and establisha sixteenth signal of the difference therebetween; differencetherebetween; means to sum said sixteenth signal and said first signalto establish a seventeenth signal; and means to compare said seventeenthsignal with said constant signal to establish said control signal. 4.The control system of claim 1 wherein said means to modify comprisesmeans to divide said control signal by said third signal and establish asixth representative of the quotient; and means to combine said sixthsignal with said fourth signal and establish said fifth signalrepresentative of the resulting difference.
 5. The control system ofclaim 4, further comprising means to establish a seventh signalrepresentative of the rate of change of catalyst within said reactor;and means to compare said sixth signal with said seventh signal andestablish a signal representative of the difference therebetween, saidlast-mentioned signal being compared with said fourth signal toestablish said fifth signal.
 6. The control system of claim 4 whereinsaid means to establish said control signal includes means to integratesaid control signal, and means to sum said constant signal with theresulting integrated signal to establish said control signal.